The first step to calculate phonons in a crystal is to construct the
super-cell, this can be done using the following setting:

LSUPER =.TRUE. (default)

NDIM = NX NY NZ (default: 1 1 1)

NTYPES = 2 (no default)

where NX, NY and NZ are positive integer numbers. NTYPES is the number of different atomic species (2 in the Stishovite example). PHON reads the file POSCAR which must contain a super-cell, (possibly ) and generates the file SPOSCAR which contains the super-cell whose lattice vectors are , where are the lattice vectors of the primitive cell. Do not use NX NY if the crystal has a symmetry operation which connects with . For face-centred-cubic (FCC) and body-centred-cubic (BCC) crystals do not use simple-cubic lattice vectors with 4 and 2 atoms in the primitive cell respectively, use proper FCC and BCC lattice vectors and one atom per primitive cell.

The program prints out a guess for the displacements needed to
construct the full force field. These are written on the stout and in
the file DISP. The size of the displacement can be changed setting the
variable DISP:

DISP = 25 (default, corresponding to displacements of 0.04 Å).

In principle, a single off symmetry displacement could be enough to
generate the full force field, as three linearly independent
displacement may be generated from this using the symmetry operations
of the crystal (this may not be true if the crystal has low
symmetry). In practice, however, one would like to use displacements
that alter as little as possible the symmetry of the crystal, because
this minimises the computational effort in *ab-initio*
calculations (maximising the number of symmetry operations in general
reduces the number of **k**-points needed). For this reason, the
displacements suggested by the program work on the assumption that the
first displacement will be along the -axis, and appropriate
additional displacements along and/or are suggested if they
are needed. The user may suggest his own first displacement using the
variable: DXSTART:

DXSTART = xstart ystart zstart

To use central differences (eqn. 4 instead of eqn. 3), set the variable LCENTRAL = .T.